We used Tiffany last week round trip and had a great experience with the reserved grocery stop as well, I spent 30 minutes and $70 worth of breakfast and snacks for the week. With the coupon, I believe it comes out to $75 round trip so depending on the number in your party, it might turn out cheaper, since Mears charges per person.
It was really nice not to have to worry about sharing our vehicle with anybody else. Our driver was waiting at baggage claim for us (waited an hour for our delayed arrival) and handled our luggage and the car was right there and ready for US and OUR schedule.
From most of the posts I have read on these boards, I have heard many great reviews as the norm for Tiffany and Yourride, which a few minor exceptions for each. Either would probably be a safe bet.
Haven't heard overwhelming positives about Mears but it is a different service.
We found with 3 small children, a towncar service was the perfect thing to keep everybody happy after a long flight and then a magical week.
Hope this helps,
Tammy